The present invention relates generally to coherent receivers, and more particularly, to a pilot-based time domain phase noise mitigation for a coherent receiver.
One main challenge with using a coherent detector is its sensitivity for laser phase noise (PN) and nonlinear impairments. Laser phase noise is caused by the uncorrelated local oscillator (LO) laser. The unstable wavelength difference between the LO and the signal behaves like a random phase noise for the received signal of the coherent receiver. The fiber non-linearity phase noise is mainly caused by the self-phase modulation (SPM) and Cross-phase modulation (XPM). Those distortions will contribute both to the phase and amplitude of the received signal. The overall phase noise at the coherent receiver is one of the major limitations of transmission system.
In one prior effort, an orthogonal basis expansion based method to suppress both common phase error (CPE) and the partial ICI for CO-OFDM systems is proposed. However, this proposed method requires an accurate channel matrix to reduce the phase noise which may not be available before the phase noise is completely removed. Other prior works used pilots to estimate phase noise in frequency domain, but they could only find CPE. In another prior work, an RF tone pilot was used to reduce the phase noise within one OFDM symbol, but there was some residual phase noise left. A further prior work used optical phase lock loop to reduce the phase noise, but the system complexity and cost would higher due to the feedback circuit.
Accordingly, there is a need for a phase noise mitigation for a coherent receiver that improves on the shortcomings of prior efforts.